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KVM switch circuit / Multi-pole double throw switch

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I've been searching internet, using all manner of phrases to find a circuit diagram for what should be a pretty simple project. What I'm hoping to build is a KVM (Keyboard / Video / Mouse) switch circuit for my computer.

A single keyboard, monitor, and mouse plug into the circuit, and the circuit is connected to the corresponding ports of two separate computer towers. Normally computer #1 sends/recieves the input/output to the peripherals. Flip a single switch, and the inputs/outputs are directed to computer #2.

In essence, a multiple-pole, double-throw switch.

I have asked around, and been told that multiplexing ICs are the key. I have several DM74ALS157 ICs on hand, and have attempted to build a simple circuit toggling between two LEDs using the attached diagram, but I've been unable to get a reaction out of it.

My electronics knowledge is sporadic; I can follow along in some technical discussions, and while I might not always understand how something works, I have no trouble following diagrams.


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switching and multiplexing chips are highly dependent upon the impedance of the circuit. Something like the CD4053 is pretty common for the family of CMOS switches. Any answer you get here will depend upon the rest of the circuit which you have not posted. Ohm out the devices you are switching to see what your impedance is and look at the data sheet for the chip you are using. This will give you a clue as to why it is not working.


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The 157 is not a bidirectional switch

I had begun to wonder about that. In which case the video portion of the circuit would presumably work (two inputs, only one selected to output) while the keyboard and mouse would not (one input to one of two outputs).

I'm by no means restricted to using 157 chips. Getting parts is not a problem -- finding designs are.
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How are you getting bidirectional from this snippet of schematic? I do not see that. I see input and output clearly stated. Author states he is trying to light LEDs with the switch.


The 4053 is an analog bidirectional switch.
Looking at the pinning of the 157 it is easy to make the assumption that it is too. The OP saying that he was trying to switch between 2 leds lead me to the possibility that he had made the same.
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  • 2 weeks later...

Dear All,

I just hope we get this topic as much as rich as possible coz i dont think there are any other good topics on KVMS elsewhere.

First, i think that a 4066 CMOS switch could be used to switch the mouse and keyboard. Now the main problem is how to trick the PC that a mouse is still connected to it. I am not sure what should we hold is it the voltage? or what. I would really like to see a detailed schematics for such KVM. Plus i am not getting the problem with switching the monitor pinouts using such CMOS switch. Please help in making this topic fruitful and updated.

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If PS2 Keyboard & Mouse are used then The device provides a clock & a data line (similar to I2C check http://www.networktechinc.com/ps2-prots.html.) So we have to switch only the data line. By buffering the clock line to both PC's we can insure that PC assumes a device is present on that port. But the problem dose not end's here. The switching must be done when the device is not transmitting anything to PC. So probably a microcontroller should be used to receive signals from kbd & mouse and route them to appropriate PC. That way one can impliment Hotkey for switching from one system to another. As far as video signal is concerned I would like to see a post from a person who has done video multiplexing. So all ye Electronics Video God's out there plz listen to the prayer of this child & shower us with video mux circuits ;D ;D ;D

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Here. Throughout of my research on the web, i probably found the ideal source for our KVM switch project. Yet i think we need someone to read it and write a schematics from it. I ran into the thesis projects of two students. the first student was working on switching keyboard and mouse. and the second student was working on the video switching. Take a look here.



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